1. Field of the Invention
This invention relates to a surface-pressure distribution sensor and a method for controlling the operation of the sensor and more particularly to a surface-pressure distribution sensor capable of reducing power consumption and a method for controlling the operation of the sensor.
2. Description of the Related Art
A surface-pressure distribution sensor that uses a flexible conductive film and a TFT (Thin Film Transistor) is known as a device for detecting a fine concavo-convex pattern like a fingerprint pattern (Japanese Patent Application Publication No. Hei 6-288845).
FIGS. 10A–10C show an example of an active matrix type surface-pressure distribution sensor for detecting a fingerprint pattern. FIG. 10A is a plan view, and FIGS. 10B and 10C are sectional views along line C—C of FIG. 10A.
A conventional surface-pressure distribution sensor 200 includes a glass or ceramic board 201, on which a plurality of TFTs 204a serving as unit detection elements are disposed, and an counter electrode film 202.
The unit detection element 204 includes the TFT 204a and a contact electrode connected to the TFT 204a. The unit detection elements 204 are arranged in a matrix form on the board 201 made of glass or the like. The active layer of the TFT forming the unit detection element 204 is an amorphous silicon film, and the contact electrode 204b is made of ITO (Indium Tin Oxide).
The counter electrode film 202 is disposed to face the board 201 and has a conductive film 202b deposited on the back face (on the side of the TFT) of a flexible insulating film 202a. 
The counter electrode film 202 is fixed by a sealing agent 203 applied to the periphery of the board 201 and is spaced apart from the board 201.
An example of a method for manufacturing the surface-pressure distribution sensor is as follows. After TFTs are formed on the board 201, a sealing agent 203 of low-temperature thermosetting resin is applied to the periphery of the board 201 in order to adhere the counter electrode film 202. Thereafter, the counter electrode film 202 of the board 201 is adhered thereon and is subjected to thermal treatment. As a result, the board 201 and the counter electrode film 202 are fixed together.
FIG. 10C shows an example of detecting a fingerprint pattern by use of this surface-pressure distribution sensor.
The entire electrode film 202 is pressed down by lightly pushing the surface-pressure distribution sensor 200 while placing a finger F thereon. However, since a difference in pressing force exists between a mountain part and a valley part of the fingerprint in minute scale, a contact electrode 204b of a unit detection element 204 disposed exactly under the mountain part or disposed in close vicinity of the mountain part comes into electric contact with the counter electrode film 202. However, the contact electrode 204b of the unit detection element 204 disposed exactly under the valley part of the fingerprint or disposed in the vicinity of the valley part does not come into electric contact with the counter electrode film 202. Thus, a signal of a portion where the electrode film 202 and the unit detection element 204 come into contact with each other is picked up to detect a fingerprint pattern.
As described above, in the conventional surface-pressure distribution sensor, a sensor area is scanned while always applying scanning signals from a drain wire and a gate wire. For example, a display device such as an LCD (liquid crystal display) needs to scan all the time for the purpose of displaying, whereas the surface-pressure distribution sensor used to detect fingerprints only has to operate at least when the fingerprints are detected. Additionally, the TFT is always kept driving because of the continuous scanning, thus causing fast deterioration of the TFT and hindering its life from being made longer.